Tubelight at home randomly stopped working this week for no apparent reason. Opened it up to see what went wrong. Started probing and saw that the input fuse (Supposedly a fuse but they put a 61Ohm resistor) was blown. Replaced that and the 41Ω, 1W resistor on the return path, and the tube light was back to normal. Not sure what caused the blow-up. Maybe the resistor degraded with time.
I went through and traced out the circuit for anyone wanting to know its schematic. The AC Input side is current limited with a “fuse” and fed into a classic full wave bridge rectifier(MB105) IC to convert to high voltage DC. There is space given in the input for filtration/safety capacitors but they have not populated it. Probably to save a few cents, might have put it there to pass the testing. The output of the rectifier contains a CY400 capacitor and Metal Oxide Varistors(MOV). This is fed to SIC9762 which is a high voltage LED driver with an integrated MOSFET. It’s a constant current LED driver with the drive current set at around 200mA with a sense resistor. On the output LED side, you have a large electrolytic capacitor of 100uF with a bleeding resistor to discharge the tube light immediately(else you will get a ghosting effect of the LED). There are 100 LEDs in total at the output.
All in all, it’s a penny-pinched circuit optimized to minimize the BOM to the lowest price possible. That’s exactly the reason you get these tube lights in the $3-$4 range per unit in the retail market.